1. Field of the Invention
This invention deals with light modulators, more particularly with addressing and timing techniques for systems using these modulators.
2. Description of the Prior Art
Binary light modulators possess two states. One state, corresponding to a "zero", transmits no light. The other, corresponding to a "one", transmits light at the maximum intensity for whatever system is under consideration. In short, these modulators are either OFF or ON. As a result, only two discrete light levels exist at the viewer's eye, black and maximum brightness. Intermediate levels during pixel on/off state changes are ignored as they are of relatively short duration. To achieve intermediate (similar to analog) levels of light as perceived by the viewer, pulse-width modulation (PWM) techniques are employed.
The basic PWM scheme is as follows. The rate at which the analog images are to be presented to the viewer is determined. This establishes a frame rate (frequency) and a corresponding frame time. In a standard television system, for example, images are transmitted at 30 frames per second, and each frame lasts for approximately 33.3 milliseconds.
The intensity quantization for each sampled point in the frame, or image, which maps to one picture element (pixel), is established. Assuming 6 bits of quantization, this means 1 part in 64, 63 of which are non-zero. In this example, the frame time of 33.3 msec is divided into 63 equal time slices. The resulting time slice, or least-significant-bit (LSB) time, equals 33.3 msec/63 or 528.6 microseconds.
Having established these times, all possible analog pixel intensities as seen by the viewer are scaled and quantized, such that black is 0 time slices and maximum brightness is 63 time slices. The scaled and quantized intensities set the on-time for the pixel so it is ON for the corresponding number of LSB times. In the simplest case, all pixels with a quantized value more than zero are turned ON at the beginning of a frame time, and they remain ON for the number of LSB times which correspond to their relative analog intensity. The viewer's eye will integrate the points of maximum intensity so that it would appear the same as if they were a temporally constant analog level of light.
The maximum burst bandwidth requirements for a binary modulator High Definition Television (HDTV) display using this scheme can be calculated as follows. Assuming worst case, where all pixels in a given frame will have intensity values between black and maximum brightness, all pixels will have to change at the beginning of the next frame. The LSB time can be calculated as follows: ##EQU1## So for these values, the LSB time would be 176.3 microseconds. In this time period, 2,073,600 pixels (1920.times.1080) must be loaded. The data rate is given by the following: ##EQU2## The data rate equals 11.76 Gigabits per second. The cost of building such a system is prohibitive.
There are many ways to implement PWM to lower the effective data rate. The data can be input to the pixel in a highly parallel fashion. For example, an input shift register which is partitioned into section of 8 pixels each, with an off-chip data input to each shift register can be added. For 1920 pixels that would be 240 shift registers, sharing a common clock. These 240 shift registers could be loaded with one row of data, using only eight clock pulses. This reduces the data rate by a factor of 240, or 49.1 Mbits/second.
Additionally, the outputs of each shift register could drive a parallel data latch. This data latch stores the contents of the input shift registers after they have been filled. This allows the input shift registers to accept a new row of data while the previously latched data is stored in the selected row of the pixel array. Consequently, the pixel array will be addressed at a rate 8 times slower than that which the input circuitry is operating. This limits the high speed circuitry needed for the pixel chip.
The input shift register/parallel latch combination can be added at the top and bottom of the array. This allows the top and bottom half of the device to be addressed simultaneously. Now each set of registers/latches need only read in half the data in a given frame time. So the data rate is reduced by another factor of two. The new input data rate is 24.55 Mbits/second, but the pin count is 480.
While these architectural changes have drastically reduced the per pin input data rate to the pixel array at the expense of increased pin count, they have also imposed a constraint on the method of addressing the pixel. While the single input method is random access, the modified architectures of this type require the pixels to be addressed a row at a time.